The default DNS blacklist, blackholes.mail-abuse.org,is a service offered by the Mail Abuse Prevention System(MAPS). As of July 31, 2001, MAPS is a subscriptionservice, so using that network address won't work if youhaven't subscribed. Contact MAPS to subscribe(http://mail-abuse.org/).

This is the service which is setup by default in server.tab.So if you are not subscribed, it won't work. I suggest to use the following which are still for free and work good: